Aircraft control apparatus



Dec. 2, 1952 F. P. sTRoTHER 2,620,149

AIRCRAFTV CONTROL APPARATUS Filed Dec. 5, 1947 Lttorneg Patented Dec. 2, 1952 AIRCRAFT CONTROL APPARATUS Fred P. Strother, Riverside, Conn., assignor to `Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application December 5, 1947, Serial N0. 789,831

18 Claims. 1

My invention is an improvement in apparatus for controlling an aircraft in `flight. 'An apparatus embodying my invention coacts with a control surface which positions the aircraft about one axis thereof and coacts withthe power means for propelling the aircraft.

An object of my invention is to provide in such apparatus means responsive to variations of the aircraft from al selected altitude'and from a selected air speed for jointly controlling the operation of the control `surface' andthe power of the propelling means.

A further object of my invention is to provide 5in such apparatus having altitude and air speed responsive means two balanceable systems so that fthe control applied to-said control surface and said power means is proportional to the extent of variation in altitude and air speed.

A further object of my invention is to provide 'devices responsive to changes in altitude and air speed of the aircraft which coact with a pressure responsive device tending to maintain nearly constant the power of the propelling means of an aircraft and also which coact with a device responsive to movement of the aircraft about an axis which tends tomaintain the attitude ofthe aircraft through a control surface.

A further object of my invention is to provide in such apparatus having altitude land air speed responsive means for controlling a control surface of said aircraft and power means for propelling said aircraft, means responsive to the tilt of the aircraft about an axis to modify the position of the control surface during changes in altitude or in air speeds so that a compensating effect may be applied to said aircraft through said control surface while corrections for changes in altitude or air speeds are also applied through said power means.

A further object of my invention is to provide a joint control of the control surface of. an aircraftwhich controls the position of the aircraft about an axis, by an altitude responsive device and by an air speed responsive device.

A further object of my invention is to provide a device responsive to the movement of the aircraft about two axes and devices responsive to changes in altitude and air speed of the aircraft for controlling a control surface and power means whereby for incurred changes in altitude or air I speed the device responsive to the movement of the aircraft about two axes modifies a compensating operation ofthe control surface while corrections for the change in'altitude and air speed are applied to the power means and wherein said 2 altitude and air speed responsive devices may modify the displacement of the control surface as initiated by the axis responsive device upon movement of the aircraft about a second axis.

Further objects of my invention will be apparent by reference to the accompanying description 0f an embodiment thereof which is illustrated in the drawing. The particular form of the apparatus to be described merely illustrates one arrangement for effecting my invention. It is not necessarily limited thereto but may be measured by the statements of the claims.

The sole gure of the drawing shows the preferred arrangement of the apparatustembodying the invention. v

In said arrangement, the power means or aircraft engine (not shown) hasiits carburetor connected to a carburetor intake line I0 similar to the arrangement provided byD. G. Taylor in Patent 2,388,350, dated November 6, 1945. The power developed by such aircraft engine depends upon the weight of air and fuelmixture supplied in any given time. The weight of air and fuel mixture supplied to the engine depends upon the pressure within the line I0. The line I0 is supplied with air under pressure bya variable speed air compressor I2. The compressor I2 is driven by an exhaust turbine I4. lThe speed of the turbine I4 which determines the speed of compressor I2 and the resulting pressure within the line I0 is controlled by a valvev I9 'designated a waste gate.

Thus as the valve I9 is adjusted to vary the pressure in line I0 the engine power varies. The valve I9 is operated by a reversible electric motor 30. The motor 30 may be of the reversible twophase type similar to that disclosed in the aforementioned patent and is controlled by an amplifier 40. l

The amplifier 40 has an 'input circuit which is controlled by an intake line pressure responsive element 63 which tends to nmaintain nearly constant the pressure within the carburetor intake line I0 as the aircraftfmoves' from ground position to high altitudes.

Amplifier 40 is also controlled by a selector 88 by which the pressuresfwhich are to be approximately maintained in line In()Y may be selected. 4 1

Further control of amplifier I0` isv obtained by an altitude responsive devicel I!! which so rcontrols amplifier 40 as to causera reduction in pressure in line I0 as the desired altitude is exceeded.

An indicated air speed responsive device |38 also controls amplifier 40 in such manner as to decrease the pressure within intake line I8 if speed above that desired is exceeded.

The control surface which in this invention illustrated is the conventional elevator, not shown, of an aircraft is operated from cables |65 from a cable drum |6|. The cable drum |6| is driven by a motor |64 which is reversibly controlled by an amplifier |65.

The amplier |65 includes an input circuit which is controlled by a vertical flight gyro |91?. to stabilize the aircraft about the pitch axis.

Amplier |65 is also controlled by the altitude responsive device H6 which so controls the amplifier as to cause the downward positioning of the elevator if the plane exceeds the desired altitude.

Amplifier |65 has its input also controlled by an air speed responsive device |36 which so controls amplier |65 as to cause the application of down elevator if less than the desired air speed is obtained. For excesss air speed, the amplifier |65 is so controlled as to apply up elevator.

The vertical flightgyro |94 also controls amplilier |65 so as to apply up elevator when the airf craft is banked.

Having given a general impression of the operation of the embodiment illustrated, the specic details thereof will be described. Since the intake pressure, ltherefore developed by the aircraft engine is controlled by the motor 38 which positions the waste gate I9, the control elements associated with motor `3|l will be described rst. As previously notedinotor 36 is controlled by amplifier 40.

The direction of rotation of motor 36 depends upon the instantaneous phase relationship between the line voltage or that supp-lied to motor 38 and amplifier 40 and the voltage across the signal input terminals connected to leads 50 and |3| of amplier 40. The phase and magnitude 'of the voltage across the terminals of amplifier 40 connected toleads 58 and |3| is determined by a circuit which includes lead 50, rebalancing-intake pressure network 56, lead 16, turbo boost selector-calibrating impedance network 86, lead 98, altitude-airspeed impedance network H6. lead |30, to ground and to grounded lead |3| and to the amplier. The series connected impedance networks 56, 80, and which are included in the input circuit to amplifier 28 are in themselves sources of signal voltage. The phase of the voltage across the terminals connected to' leads 50 and |31 of amplifier 40 is dependent upon the algebraic sum of the voltages in the separate networks.v In response to an initiating signal in any one of the impedance networks 56, 80, or the amplifier 40 causes the motor 38 to operate to position the `valve I6.

'I'he rebalancing-intake pressure impedance network 56 comprises a carburetor intake pressure responsive potentiometer having a resistor 51 whose opposite ends are connected respectively by means of leads'll and 6| to the corresponding ends of a secondary winding 62 of a transformer. The transformer is provided with a primary winding 2U'. Since in the several networks to be described, the individual secondary windings thereof may have a common primary winding, the reference character, 20, is used to indicate the primary windings in each instance.

'Ihe primary winding20 is supplied with alternating current from a supply which may be the inverter (not shown) of the aircraft. A wiper 58 bears upon the surface of resistor 51 and it is operated by an operating connection 64 extending from a dividing wall 65 of a double bellows 63 4 which constitutes the intake line pressure responsive device.

The double bellows device 63 includes a right section 66 and a left section 61 and they are interposed between two xed frame elements, as shown. The left bellows 61 has been evacuated and a conventional spring (not shown) is mounted within the bellows'61 and exerts a pressure upon the dividing wall 65. The right bellows 66 has pressure applied thereto from a lead 66 extending from the carburetor intake line SQ. Any dierential pressure between the pressure exerted by the spring on the dividing wall 65 and that exerted upon the wall 65 by the pressure within the carburetor intake is applied through the operating connection 64 to wiper 56. Due to the fact that atmospheric pressure acts equally in opposite directions upon bellows 66 and 51, the Wiper 58 is not aiected by changes in atmospheric pressure.

Impedance network 56 also includes a sorvomotor rebalance potentiometer comprising a resistor 5| whose opposite ends are connected to the corresponding ends of secondary winding 62 and a wiper 52. The'wiper 52'is operated by a follow up connection 53 from servomotor 38. A lead 58 extends from one signal input terminal of ampli.- fier 40 to the wiper 52 of network 56.

In the network56, if at a particular instant the right end of secondary winding 62' be consideredpositive with respect to its center then the left end of the secondary winding is negative with respect to its center. The right end of resistors 5| and '51' would have the same polarity as the right end of secondary winding 62, also the left end of resistors 5| and 51 would have the same potential as the left end of secondary winding 62. If the wiper 52 be 'moved from the right end of resistor 5| towards the left end of resistor 5| it may be rseen that the potential of wiper 52 with respect to the center of winding 5| varies from a positive value and decreases until the center of the resistor 5I is reached. As the wiper 52 is moved to the left from the center of resistor 5| its potential with respect to the center of resistor 5| increases but is of opposite phase. In other words the Wiper now assumes a negative potential with respect to the center of resistor 5|. In the position shown the wipers 52 and 58 are at the centers of their respective resistors 5| and 51 and there is no potential difference between them.

Assuming again that the right end of secondary winding 62 is positive with respect to its center and if wiper 58 now be vmoved to the right of its present position with wiper 52 at center, the wiper 58 will be positive with respect to the wiper 52. On the other hand if wiper 58 be moved to the left from its position shown, its potential with respect to wiper 52 increases but is of opposite phase from the potential developed when wiper 58 was moved to the right. It is therefore seen that the relative position of wiper 5B with respect to wiper 52 determines the phase of the voltage signal developed by network 56.

The turbo boost selector-Calibrating impedance network includes a turbo boost selector potentiometer having a resistor 8| whose opposite ends are connected by means of leads 82 and 83 to the corresponding opposite ends of a secondary winding 84 of a transformer having a primary winding 20. A wiper 86 of the potentiometer is driven by an operating connection 81 extending from a turbo boost selector 88. A manually operable knob 89 of the turbo boost selector 88 pro- 5. vides the means for driving'connectionl. The knob 89 has a pointer which coactsfwithindicia Afor indicating the amount ci pressure to 'be maintained in intake I0. A lead 10 extendsvfrom wiper 86 of the turbo boost selector potentiometer to wiper 58 of the intake pressure responsive po'- tentiometer wiper 58.

The impedance network 65 alsoincludes acali'- brating or centering potentiometer Whose resistor 6l] has its ends connected to the corresponding ends of secondary winding 84. A wiper 9| of the centering potentiometer is manually operable by a centering knob 62. The network 80 is similar to the network 56 being'in-a form of a wheatstone bridge whose output is derived from a voltage dinerence between wipers 86 and 9|. In the positions shown, the wipers'86 andlf-ar'e at the mid-points of their-respective resistors 6| and 96. The mid-points or the resistors 8| and 9|) like the mid-point of secondary wniding 84 is for the purpose under consideration considered the electrical neutral point. With wipers 86 and 9| at their electrical centers of their respective resistors 95 and 8| there isno potential difference across wipers 86 and 9|.` Ii one wiper be moved from its electrical center Vit will'have a potential with respect to the other wiper. Similarly if at any time the twowipers have a different displacement from the electricalcenters of their resistors, there will be a potential across the wipers 86 and 9| The altitude-air speed impedance network IID includes an altitude responsive potentiometer having'a resistor I I whose opposite ends are connected by means of leads ||2 and |I3 to the corresponding opposite'endsof'asecondary winding I I4 of a transformer having a primarywinding 25. A wiper ||5'of the altitude responsive potentiometer is electrically connected by means of lead 58 to wiper 9| -of .the centering potentiometer of network 86. Wiper I|5 is operated by a connection I |1l extending from a dividing wall H6 of the double bellows altitude sensing arrangement ||9.

The altitude sensingu device H9 vcomprises a bellows |25 and a bellows |24 having a dividing wall ||8 between them. The bellows |26 and |24 are interposed between nXed frame members. Static pressure or atmospheric pressure is led through an open tube |2| to the interior of bellows |24. A tube |22 communicates with the interior of bellows |20. The tube |22 is 'connected to a valve |23 whereby the bellows 4'|20 may be selectively placed in communication rwith the atmosphere. y same atmospheric pressure is applied to both bellows |25 and |24 and the dividing'wall' I8 will have equal pressures applied Von its side. In such case no movement will be given to wall IIS. If valve |23 be closed vand if thereafter the atmospheric pressure should change the'pressurewithin the bellows |25 and |24 would be unequal and the dividing wall IIS having l"differential pressure applied thereto would moveinfthe diirection of the bellows having the smaller pressure. I

The impedance network I lll also includes an air speed. responsive potentiometer whose resistor |51 is crossconnected by means lo leads |33 and |34 to the ends of secondary winding l I4. Instead of having the corresponding ends of resistor |3| connected to the ends oi secondary winding ||`4 as has been done with the altitude responsive resistor the leads |33, |34 are crossed as shown to provide the proper phasing of the voltage When valve |23 is open -the across the resistor. A wiper |32 of the air speed responsive potentiometer is connected to one end of a resistor of a trimmer potentiometer |35. The opposite end of the resistor is connected to a center tap of resistor |3|. A lead |30 connects a Wiper of the trimmer potentiometer to ground. The trimmer potentiometer |35 may be adjusted whereby the ratio of the signals from network ||0 and network 23|), to be described, due to changes in airspeed may be varied. Thus a primary signal on changes in air speed is applied to an elevator motor amplier input circuit and a secondary signal may be applied to the waste gate motor amplifier input circuit. The wiper |32 is operated by a connection |36 extending from a dividing wall |31 of the air speed response of device |38.

The air speed responsive device |38 comprises a double bellows having sections |4| and |42. The two sections are separated by the dividing wall |31. The sections |4| and |42 are interposed between xed frame members. A tube |44 connects with the interior of section I 42. The tube |44 which communicates with the interior of section |42 has pressure applied thereto proportional to the air speed of the aircraft. A tube |45 communicates with the interior of section |41. The tube |45 also connects with a valve |55. A tube |48 extending from the valve |50 has pressure applied thereto proportional to the airspeed of the craft. The valve |50 mayY be'selectively closed to discontinue the'v transfer of pressure proporti'onal to air speed from tube 481to the section I4I. The bellows section |4| `may therefore be rendered non-responsive to pressures from the air speed device whereas the bellows section |42 is always responsive to the air speed device 'communicating with tube |44.

The elevator control surface of the aircraft is operated by cables previously'mentionedY eX- tending from a servomotor cable drum I 6|. The cable drum is driven by an operating connection |62 extending from a servomotor |64 which may be of the type disclosed in application 447,989 i'iled June 22, 1942 or in the patent to W. H. Gille et al. 2,425,733 issued August 19, 1947. y Such servomotor is powered by a D. C. motor which may be energized by a battery |63.

The servomotor |64 reversibly drives connection |62 as determined by an elevator amplifier |65. The'arnplifier |65 is connected Ytoa source of alternating current which may be the inverter of the aircraft. The direction of rotation applied to connection |62 by motor |64 depends upon the instantaneous phase relationship between the voltage. supplied by the inverter to the amplier I65'and the voltage across the signal input terminals of the amplier which are connected to leads |10 and 25|. l

`The ampliner input control elements are connectedin a signal circuit which extends from lead |10, a rebalancing-vertical gyro pitch impedance Vnetwork |80, lead |95, a vertical'gyro roll-centering impedance network 2 |0,`lead 224, an altitudeairspeed impedance network 236, lead 250, to ground, and to'grounded `lead 25| of amplier |65. L

The,impedance network comprises a servobalance potentiometer having a resistor i6 I whose vends are connected by means of leads |82 andi to the corresponding'ends ,of secondary winding |84 of a transformer havingV a primary winding 21|.'V -fI'he primary Awinding 26 as previously mentioned isconnected to the inverter .of lthe aircraft 'o'r other source of valternating current, not shown.

The wiper |86 of the servobalance potentiometer is electrically connected by means of lead to one input terminal of amplifier |65. The wiper |36 is operated by the elevator servomctor |64 through the operating connection |02, constituting a follow up arrangement. The impedance network |80 also includes a vertical gyro pitch axis potentiometer whose resistor |90 is connected across the secondary winding |84. A wiper |92 of this potentiometer is operatively connected to a vertical flight gyro |94 by means of an operating connection |93. The vertical flight gyro |94 may be of the type well known in the art having a rotor whose axis of rotation is vertical and in which movement of the plane about the pitch axis causes a proportional relative movement between the operating connection |93 and the aircraft. Since the resistor |90 is carried by the aircraft, a relative movement will therefore result between wiper |92 and resistor |90 when the craft moves about the pitch axis. Wipers |36 and |92 are normally at the electrical centers of their respective resistors IBI and |90. When the wipers are so positioned, there is no potential difference across the wipers |80 and |90. In network |89, as in previous networks, a voltage signal proportional to the potential difference across wipers |33 and |92 is developed whenever the wipers |86 and |92 are differently displaced from the electrical centers of their resistors |8| and |90.

The vertical gyro roll axis-centering impedance network 2|0 comprises a roll axis responsive potentiometer whose resistor 2|| has its ends connected by means of leads 2|2 and 2 |3 across a secondary winding 2|4 of a transformer having a primary winding 20. A wiper 2|5 of the roll axis potentiometer is connected by means of lead |95 to wiper |92 of the pitch axis responsive potentiometer. The wiper 2 |5 is operatively driven by a connection 2|6 extending from the vertical iiight gyro |94. The vertical flight gyro |94 through the operating connection 2|5 stabilizes the Wiper 2|5 upon movements of the aircraft about the roll axis. The resistor 2| on the other hand is carried by the aircraft consequently upon movement of the plane about the roll axis. The wiper 2|5 and resistor 2|| will have a relative displacement. The impedance network 2|0 also includes a centering potentiometer whose resistor 2|8 is connected across the secondary winding 2 |4. The centering potentiometer wiper 2 9 is manually adjusted by a knob 22|. Network 2 I0 may develop a signal voltage when wipers 2 l5, 2|9 are relatively displaced from the centers of their respective resistors.

The altitude-airspeed impedance network 230 comprises an air speed responsive potentiometer which has a resistor 23| connected by means of leads 232, 233 across a secondary winding 234 of a transformer having a primary winding 20. The air speed responsive potentiometer has a wiper 235 which is operatively driven by means of connection |36 from the dividing wall |31 of the air speed responsive device |38. The lead 224 extends from wiper 235 to wiper 2|9 of the centering potentiometer. The impedance network 230 includes an altitude responsive potentiometer which has a resistor 231. The opposite ends of resistor 231 are connected to the corresponding ends of secondary winding 234. A wiper 238 of the altitude responsive potentiometer is operatively driven by the dividing wall ||8 of the altitude sensing device |9 by an operative connection ||1. Wiper 238 is electrically connected to ground by lead 250. Wipers 235 and 238 are normally at the electrical centers of their respective resistors 23|, v231. If the wipers 235 and 238 be relatively displaced from the electrical centers of their resistors in the same direction a potential difference will exist across the wipers 235 and 238. The magnitude of the potential difference between wipers 235 and 238 depends upon the differential movement of the two wipers. The phase of the voltage across the wipers in a given half cycle depends upon which wiper has been moved the greatest from the electrical center of its resistor. If in the half cycle considered, the upper end of secondary winding be at a positive potential with respect to the center and the wipers be moved in opposite directions from the centers of their resistor the phase depends upon which wiper is nearer the positive end of secondary 234. A trimmer potentiometer 239 has its resistor connected across wiper 238 and a center tap on resistor 231. Lead 250 connects the trimmer potentiometer wiper to ground. The trimmer potentiometer Wiper may be adjusted to vary the ratio of the altitude signal in the network 0 with respect to the altitude signal in network 230. Thus a primary signal on change in altitude may be applied to the waste gate motor amplifier input circuit and a secondary signal may be applied to the elevator motor amplifier input circuit.

Operation While the aircraft is still on the ground and prior to take-off, the aircraft engine is given a -ground test. The operation of the engine at this time is designated the engine run-up test. When full throttle is applied to the engine during the run-up test, the manifold pressure of the engine will depend upon the altitude of the place where the engine is being operated. If this manifold pressure does not correspond with that in accordance with the altitude of the place, an increase in the manifold pressure may be due to the fact that the waste gate |9 is partially closed. Initially the turbo selector 89 had been moved to the 0 position. If the pressure of the manifold is above that for the altitude of the place the waste gate may be toward closed position as stated so the centering knob 92 may be adjusted until the proper pressure is obtained.

With the manifold pressure as determined by the carburetor intake pressure at the proper value, the aircraft is ready for the take-off. The selector 89 is now adjusted to provide the proper manifold pressure for take-off.

The valve |23 in the altitude responsive device ||9 is open so that tubes |2| and |22 apply the same pressure to the dividing wall ||8 so that this wall is not moved. In the air speed device |38, the valve |50 is also open so that the pressures of equal value are conveyed by tubes |44 and |48 to the opposite sides of the dividing wall |31 so that the wall can not move. During the period in which the plane leaves the ground and until it becomes air borne the elevator is operated by a manual controller not shown. At this time the vertical flight gyro with its associated network may be rendered ineffective to control amplifier by opening the control circuit connected to leads |10 and 25| by a switch |1|.

After the aircraft has reached the desired altitude and air speed, the turbo boost selector knob B9 is adjusted so that proper or desired manifold pressure is obtained by adjusting the pressure in the carburetor intake line I0. The input circuit to amplifier |05 is now closed at switch |1| there- 9 by placing said circuit'under control of the vertical flight gyro |94 and other circuit control elements.

If the plane now inclines upward and thereby tends to change altitude after having been set in straight and level flight by the manual controls itis possible that the ampliiier input circuit was unbalanced at level position whereby motor |64 had moved the elevator out of streamlined or normal position. 'Ihe centering knob 22| of network 2|0 is therefore adjusted to the right or to the left whereby amplifier |65 causes the motor |64 to again rotate and bring the, elevator in streamline position for automatic control at level flight. After the plane has been placed in a level flight position through the control apparatus by the operation of the centering knob 22|,of network 2 0, the knob is left in this position and the aircraft will be maintained in level position by the vertical flight gyro |94.

The manner in which the centering knob 22| is initially operated to return the aircraft to level flight may be apparent by reference to the figure. For example, suppose that afterjtheplane has been manually controlled to thegdesired altitude that the control of the craft be transferred to the automatic control as stated. If under the automatic control, the airplane should begin to nose upward and to gain altitude theoriginal altitude and level position may be regained by operating center knob 22|. l i g Suppose at this time that the right end of secondary winding 2 I4 is positive withrespect to the left end and wiper ,92, be to the right of center due to the upward i tilt of the aircraft causing the vertical gyro to respond to the tilt. If wiper 2|9 isv moved to the right of wiper 2|5 it will be positive with respect to wiper 2 |75. Wiper 2|9 is connected to network 230 to ground and to the terminal of amplifier |65 connected to the ground and lead 25|. The wiper 2|5 is connected to leads |95, |89, lead` to the other input of amplifier |65. The amplier |65 now receives a negative signal. The amplifier |65 thereupon operates and causes the servomotor |64 to move its wiper |86 to the` right therebythe potential on wiper |86 becomes greater than potential on wiper |92. In other words wiper |86 is positive with respect to wiper |92. 4 I'he difference of potential between wipers |86 `and wiper |92 is equal and opposite to the difference of potential between wipers 2|5 and 2 |9. Theservomotor |64 continues to move wiper |86 until such equal but opposite voltage is set up. The servomotor also operates the cable drum |6| to move the elevator in a downward position. When the input circuit to amplifier |65 is balancedtheservomotor |64 ceases ,to operate andthe elevator remains as positioned. I l

As the elevator causes, the aircraft'to depress the nose from its upward inclination, the vertical flight gyro responds tothe change in attitude of the aircraft about the pitch axis and moves its wiper |92 to the left thereby causing wiper |92 to become negative with respect to kwiper |86. A positive signal will now be applied upon the input circuit to amplier |65 causing the servomotor to rotate in the opposite direction and move its wiper |36 back toward center. 'I'he difference of potentia-1 between wiper |86 andvwiper |92 is now equal and'opposite to the'voltage between wipers 2|5 and 2|9. The amplifier input circuit is balanced and the elevator is in streamlined position. When the aircraft reaches the". `desired aititude, the centering knob 22| lmay beadjusted tohold this 10 altitude with the elevator streamlined. Under normal conditions when the elevator is in its streamlined position, the wipers |86, |92, 2| 5, and 2|9 are at the electrical centers of their respective resistors.

During the time that the aircraft leaves the ground and until it reaches the desired altitude the pressure of the atmosphere decreases. The decrease in atmospheric pressure causes the carburetor intake pressure inline I9 to decrease. This decrease in the pressure in line l0 is sensed by the intake pressure responsive device 63 since the pressure in section 6-6 decreases. The spring within the left section 6l ofthe manifold pressure responsive bellows moves wiper 58 towards the right end of resistor 51. Presuming the right end of the secondary winding to be positive, it may be seen that wiper 58 will be positive with respect to wiper 52. -A negative signal will now be applied to amplifier 49 and cause it to effect operation of motor 38 so that the waste gate I9 is moved toward the closed position. As the motor 39 operates valve 19, it also throughl its follow-up connection 53 positionsl rebalancing wiper 52 toward the right whereby wipers 52 and 58 are placed at the same potential. The motor 36 therefore stops operating since the input circuit to amplifier 46 is in balanced condition. The closing of the waste gatev |9 or its movement toward a closed position causes more exhaust gas to flow through the exhaust turbine I4 thereby increasing its speed; vThe centrifugal compressor |2 is operated `bythe turbine |4 from shaft and the rotating speed having been increased the pressure within the line l0 will increase. 'Ihe pressure responsive bellows 66, 61 thereby maintain the pressure within the line I!) near the desired value.`

With the aircraft at the desired altitude and air speeds, the valve |23 of the altitude sensing device ||9 is closed and the valve |56 of the air speed responsive device |38 is closed.

It may be seen that if the aircraft rises, should it encounter an upward moving body of air, that the pressure within the bellows |26 exceeds that in bellows |24. The difference in pressure in the two bellows sections |20, |24 exists because the pressure within the chamber or bellows |20 is that at which the valve |23 was closed whereas the pressure within bellows |24 decreases as the altitude increases. The difference in pressure in bellows |29 and |24 causes the dividing plate ||8 to be moved upward which through operating connection movesl wiper 238 of network 230 and wiperl l5 of network I6 in an upward direction in the figure.

. In the operation to be described, the action of the two amplifiers 40 and |65 and eachrof the amplifier input circuit control elements are taken in sequence for purpose of analysis although the actionsV probably occur simultaneously. It is contemplated howeverthat change in altitude is primarily correctedby change inpower and secondarily by change in elevator position to maintain air speed. z

. Assume at .this time that. the upper end of secondary windings 234 and |'|41are positive with respect to the'lower. ends shown in the figure. Wiper 238 will now .be -positive with respect to Wiper 285 iin network 239. Thewiper 238 with thehigher potential is connected through lead 256 and ground to the'grounded lead 25| of amplifier |65. The wiper 235 whichk is now negative with'respect towiper v238. is [connected throughlead'224,network 2 i6, lead |95, network |88, lead |18, to the other terminal of amplifier |65. It is evident that a secondary negative signal as determined also by the adjustment of trimmer potentiometer 239 is applied across the inputs of amplifier |65. A negative signal on amplifier |65 has such phase relationship to the voltage of the inverter which supplies the motor |64 and amplifier |65 as to cause the amplifier |65 to effect rotation of motor |64 in such a direction that the elevator is lowered. The motor |64 in operating the cable drum |6| to lower the elevator also drives the follow-up wiper |86 of the servo balance potentiometer so that wiper |86 is moved towards the right until the input circuit to amplifier |65 is in balanced condition.

The plane has tilted in a downward direction due to the effect of the down elevator. The vertical flight gyro |94 responds to the tilt of the aircraft and moves its wiper |92 towards the left. Wiper |92 is now increasingly negative with respect to wiper |86. So that the input circuit to amplifier |85 is unbalanced in such a direction that the preponderance of the voltage on the amplifier is positive. The positive voltage on amplifier |65 causes the operation thereof which in turn causes the motor |64 to move the elevators back toward center and the wiper |86 is moved until the input circuit to amplifier |65 is again balanced. At this time, therefore, wipers 238, |92, |86 are displaced from normal position and the elevator is in slight down position.

When the altitude responsive device ||9 through connection raised the wiper ||5 of network ||8, wiper was raised or moved toward the positive end of secondary winding I4 and wiper |32 remained in the center position. The wiper ||5 became positive with respect to wiper |32. Wiper |5 is connected through lead 88, network 88, lead '18, network 56, lead 58, to one input of amplifier 48. Wiper |32 is connected through lead |38 to ground and from ground through lead |3| to the other input terminal of amplifier 48. The amplifier 48 receives a plus signal across its input terminals which is in phase with the voltage of the inverter or other source of supply of the aircraft. The amplifier 48 now operates and causes the motor 38 to operate under the plus signal in such a direction as to rotate the waste gate |9 toward the open position. The motor 38 through the follow-up connection 53 drives the wiper 52 of the servo balance potentiometer towards the left. It is assumed that the right end of secondary winding 62 is at this time positive with respect to the left end. Therefore, the wiper 52 is negative with respect to wiper 58. The motor 38 drives the wiper 52 until such time as the voltage between wiper 52 and wiper 58 is equal but opposite to the voltage between wipers ||5 and |32 of network ||8 at which time the input circuit to amplifier 48 is balanced. The motor 38 as previously described stops operating when the input circuit of amplifier 48 is in balanced condition.

With the waste gate |8 moved toward open position, the speed of turbine |4 decreases and the air compressor |2 likewise decreases in speed. The pressure within the carburetor intake line decreases. The pressure responsive element 63 has a decreased pressure imposed on bellows 66. The spring within the bellows 61 therefore moves the dividing plate 65 toward the right. The dividing plate 65 has its movement applied to wiper 58 through connection 64 which is therefore moved to the right. Wiper 58 being moved to the right becomes increasingly positive with respect to wiper 52 which has been moved to the left previously. A negative signal is now applied across the input terminals of amplifier 48 which causes the amplifier 48 to effect rotation of motor 38 in such a direction as to move the gate I9 toward the closed position. Actually the waste gate does not reach its original position. The motor 38 through the follow-up connection 53 also moves the wiper 52 toward the right from its left position until the amplifier input circuit is balanced. Wiper 58 is also to the right of its normal position.

Since the aircraft is in a downwardly inclined attitude due to the slight amount of down elevator, the airspeed does not change because of the decrease in power resulting from slight opening of the waste gate.

Because of the decrease in power and down elevator, the plane moves toward the desired altitude. The pressure responsive device ||9 lowers the wiper ||5 whereby a negative signal is introduced into the control circuit of amplier 48 tending to close waste gate |9. As a result, the pressure in line |8 has a tendency to increase due to the fact that the waste gate I9 has been moved back toward its original position. The pressure responsive device 63 associated with intake line |8 therefore has a tendency to move wiper 58 towards the left from its right position whereby a positive signal is impressed on the input circuit of amplifier 48. This positive signal has a tendency to open the waste gate. When the plane regains its original altitude the waste gate is in its original position and wipers 52, 58, ||5 are in their original positions.

The altimeter ||9, as the original altitude is regained, moves wiper 238 back to its original position whereby the elevator amplifier |65 receives a positive signal causing movement of the elevator to normal position.

The operation of the apparatus when the craft tends to lose altitude instead of gaining is clear from the operation where the plane tends to gain altitude. On decrease in altitude dividing wall ||8 moves downwardly calling for closing of the waste gate to provide increased power and applying up elevator to prevent an increase in airspeed because of the increased power. When the altitude is regained the waste gate is moved to normal position and the elevator is streamlined.

The operation of the elevator and the waste gate just described is associated with the response of the apparatus when the altitude of the craft has been increased due to a transient condition such as the upward movement of the body of air in which the aircraft is fiying. The aircraft may have a tendency to change its altitude due to a permanent effect on the aircraft as distinguished from a temporary effect which results from an upward movement of the air. Such permanent effect may arise where the aircraft has been in fiight for a considerable period of time. The consumption of the gasoline by the motors of the aircraft results in a lessening of the weight in such craft. Since the load of the craft decreases the plane has a tendency to increase its altitude.

Where there is a permanent effect on the aircraft tending to change its altitudes as distinguished from a temporary effect the automatically responsive apparatus will function to return the craft substantially to the desired altitude and air speed. It is evident, however, were the craft brought to the desired altitude and air speed that the permanent effect would manifest itself 13 in a change in altitude of the craft. Actually therefore the aircraft would not regain its desired altitude and air speed because of the permanent effect tending to change the altitude of the craft.

To enable the craft to fly at its desired altitude despite the permanent effect tending to change its altitude the centering knob 22| may be operated whereby a negative signal may be obtained from network 2|0 if wiper 2|!) be moved to the right from its position shown. In this situation the centering knob 22| serves as a compensating means to the altitude responsive device ||9. The signal derived from network 2|0 by the displacement of centering knob 22| to the right causes a permanent positive signal to be impressed on the input circuit of amplifier |65. The negative signal derived from network 2|0 causes the amplifier |65 to effect operation of motor |64 in such a direction as to lower the elevator and to move the Wiper |86 to balance the input circuit of amplifier |65. Thus a slight down-elevator may be carried by the aircraft to compensate for the permanent effect or force tending to change the altitude. The knob 92 of bridge 80 may be adjusted to provide the pressure in line l at which air speed is maintained.

The operation of the apparatus may be considered when the aircraft tends to increase its speed. It is contemplated that changes in air speed are corrected primarily by operating the elevator from a primary signal from network 230. Network lll) provides a secondary signal in change in air speed which controls the power means to prevent change of altitude.

If the air speed increases over .that desired the pressure within bellows |42 exceeds that in bellows |4| consequently the dividing wall |31 is moved in an upward direction in the figure. Dividing wall |3'| through its operative connection |36 moves wipers 235 and |32 of networks 23|] and Il respectively in an upward direction. Assuming that the upper ends of secondary windings |4 and 234 are positive with respect to the lower ends it is evident that wiper 235 is positive with respect to wiper 238 and. wiper |32 is negative with respect to wiper ||5. Network 23|) impresses a primary positive voltage signal on the elevator amplifier |65 which positions the elevator in an upward direction.` At the same time network ||0 impresses a secondary positive voltage signal on the amplifier 4D to open waste gate I9.

Due to the up elevator the aircraft tilts in an upward direction and the vertical gyro |94 responds and sets up a signal whereby the elevator is returned toward streamlined position but attains a slightly up elevator position. The positive signal on the waste gate motor amplifier 4i) causes the waste gate motor to open the waste gate I9. The pressure responsive bellows 66, 6l respond to the decrease in pressure Within the carburetor intake Il] and cause the repositioning of the valve |9 toward its closed position. The waste gate valve I9 assumes a slightly open position from its normal position.

The purpose of the up elevator which is carried at this time prevents a loss of altitude of the aircraft and the original altitude is maintained. The aircraft flies at the desired altitude but due to the slight opening of the waste gate loses power and therefore the air speed decreases.

As the air speed decreases the dividing wall 31 is moved downwardly toward its normal position. The wipers 235 and |32 are also moved downwardly at this time.

Downward movement of wiper |32 of network I0 results in a negative voltage signal being applied through the waste gate motor amplifier 40 whereby the waste gate motor 36 moves the waste gate I9 toward its original position. Concurrently with this movement of the waste gate I9 the movement of wiper 235 causes the network 236 to apply a negative signal on the elevator amplifier |65. The amplifier |65 causes the elevator servomotor |84 to move the elevator toward streamlined position. When the craft has regained its original air speed the waste gate is in its normal position and the wipers 52, 58 and |32 in the input circuit of amplier 4i) `are in their normal pcsitions. Similarly the wipers |86 and |92 and 235 in the input circuit of elevator amplifier |35 are in their normal positions. The plane now has regained straight and level flight with the original altitude maintained and the desired air speed regained.

The operation when the aircraft tends to lose airspeed from a desired value is somewhat the reverse from the operation when it tends to increase air speed. If the air speed decreases from a desired value, the dividing plate |37 is moved downwardly since the pressure within bellows |4| exceeds that in ybellows |42. The network 236 impresses a negative primary signal on the elevator amplifier |65 which causes its servomotor |64 to apply down elevator. The down elevator causes the aircraft to increase speed. The network ||ll concurrently due to the downward movement of wiper |32 applies a secondary negative signal on the waste gate amplifier 4 whereby the waste gate motor 35 is operated to posim tion the valve I9 toward closing position. The aircraft does not lose altitude from the down elevator due to the compensating effect of increase of power obtained by the closing of the waste gate I9. As the result of the down elevator the speed of the aircraft tends to increase. As the air speed increases and approaches the original desired value the pressure on bellows |42 increases and the dividing wall |31 moves toward its original position. The dividingI wall carries with it the wipers |32 and 235 of networks |||l and 23E).

The upward movement of wiper |32 causes the network Ii] to apply a positive signal to the waste gate amplifier 49 which in turn causes the waste gate motor 30 to move the Waste gate I9 toward open position. At the same time movement of wiper 235 upwardly causes the network 230 to apply a positive signal to the elevator1 amplifier |55. The amplifier |65 causes the elevator servomotor |64 to move the elevato-r upwardly to normal streamlined position. The aircraft therefore regains its original air speed without losing altitude.

While normally the air speed of the craft does not change when. corrections are applied for changes in altitude it is nevertheless evident that if the air speed would change that the air speed meter |33 will so position wipers |32 and 235 as to maintain the desired air speed while altitude corrections are applied. For example, if the aircraft tends to increase in altitude the dividing wall H8 of the altimeter H9 is moved upwardly. This upward movement of the dividing wall causes network 23d to apply a negative signal to the elevator amplifier |65 and causes network Hd to apply a positive signal to waste gate amplifier 4ll. Down elevator is applied and the waste gate is moved toward open position. The down elevator normally prevents a decrease in air speed when correction is being made for increase in altitude. If the amount of elevator should be such as to cause the air speed to increase, dividing wall |37 of the air speed device |38 is moved upwardly causing the wiper 235 to apply a positive signal on the elevator amplier |155. This ampliiier causes the elevator to be moved toward normal position. Concurrently the wiper |32 is raised sending a positive signal on waste gate amplifier 4i] whereby the waste gate IS is moved toward open position by the waste gate motor 30. In this respect the operation of the air speed meter aids the altimeter I9 in adjusting the waste gate toward open position to reduce the power and consequently the altitude of the aircraft. By the operation of the air speed meter |38 the air speed is maintained constant while the altitude is being corrected.

In a similar manner the altimeter H9 maintains the altitude constant while changes in air speed are being applied.

It is now evident that the apparatus of this invention provides an interrelated altitude and air speed responsive means for the dual control of the engine for propelling the aircraft and an elevator surface for controlling the aircraft. By the apparatus described, a. change in altitude occasioned by a temporary condition such as a gust of air is corrected for primarily by the alteration of the power delivered by the engine. In addition the elevator surface is operated to prevent changes in air speed. In other words a change in altitude is primarily corrected by a change in engine power and a secondary operation of the elevator is provided to maintain air speed. A change in air speed, on the other hand, is corrected for primarily by operation of the elevator, and the engine power is altered secondarily to maintain altitude.

The apparatus of this invention also provides means whereby a permanent force tending to change the altitude may be compensated for so that the aircraft will fly at the desired altitude.

I claim as my invention:

l. Flight control apparatus for an aircraft having a control surface and power means for propelling said aircraft comprising: operating means for said control surface; a first balanceable control means for said operating means; a second balanceable control means for said power means; said first control means including a first unbalancing controller responsive to changes in altitude of said aircraft, a second unbalancing controller responsive to the movement of the aircraft about an axis. and a third controller for rebalancing said first control means; and said second control means including a fourth unbalancing controller responsive to changes in altitude of said aircraft, and a fifth controller for rebalancing said second control means.

2. Flight control apparatus for an aircraft having a control surface and power means for propelling said aircraft comprising: operating means for said control surface; a first balanceable control means for said operating means, said first control means including a first unbalancing controller responsive to changes in altitude of said aircraft, a second unbalancing controller responsive to the movement of the aircraft about an axis, and a third controller for rebalancing said first control means; a second balanceable control means for said power means including a fourth unbalancing controller responsive to changes in altitude of said aircraft, a fifth unbalancing controller responsive to changes in said 16 power means, and a sixth controller for rebalancing said second control means. l

3. Flight control apparatus for an aircraft having a control surface and means for propelling said aircraft comprising: operating means for said control surface; a first control means for said operating means including a first controller responsive to changes in altitude of said aircraft for initiating operation of said operating means, a second follow-up controller for limiting the operation of said operating means, and a third controller responsive to the movement of the aircraft about an axis as a result of the movement of the control surface by said operating means; a second control means including a fourth controller responsive to the changes in altitude of said aircraft, a second operating means for varying the power of said propelling means, and a fifth controller driven by said second operating means for limiting the movement of said second operating means.

4. Flight control apparatus for an aircraft having a control surface and means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for said operating means including a series of variable impedances said rst control means including a first controller responsive to changes in air pressure connected to one varia'ble impedance, a second controller driven by said first operating means and connected to a second variable rebalancing impedance, and a third controller responsive to the movement of the aircraft about an axis and connected to a third variable impedance; a second operating means for altering the power of said propelling means; a second control means for said second operating means comprising a series of connected variable impedances, a fourth controller responsive to changes in air pressure connected to a fourth variable impedance, a fifth controller driven by said second operating means and connected to a fifth rebalancing impedance, and a sixth controller responsive to changes in power of said propelling means and connected to a sixth variable impedance.

5. Flight control apparatus for an aircraft having a control surface and means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for said yoperating means including a first controller responsive to changes in altitude of said aircraft, a second controller driven by said operating means, and a third controller responsive to the movement of the aircraft about an axis; a second operating means for controlling the air pressure supplied to said propelling means; a second control means for controlling said second operating means including a fourth controller responsive to the altitude `of the aircraft, a fifth controller driven by said second operating means, and a sixth controller responsive to the air pressure controlled by said second operating means.

6. Flight control apparatus for an aircraft having a control surface and variable speed means for controlling fuel pressures to power means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for said operating means said first control means including a first controller responsive to departures of air speed from a predetermined amount and a second controller responsive to the first yoperating means; a second operating means for controlling the variable speed means; a second control means for said second operating means, said second control means including a third controller responsive to the departures of air speeds from the predetermined amount and a controller driven by said second operating means.

'7. Flight control `apparatus for an aircraft having a control surface and variable speed means for controlling fuel to a power means for propelling said aircraft comprising: a first operating means for said control surface; a rst balanceable control means for said operating means, said rst unbalancing control means including a rst controller responsive to change in air speeds from a preset value, a second unbalancing controller responsive to the movement of the aircraft about an axis, and a third rebalancing controller driven by said operating means; a second operating means for controlling the variable speed means of said propelling means; a second balanceable control means for said second operating means said second control means including a fourth unbalancing controller responsive to changes in air speeds from a preset value and a fifth controller driven by said second operating means.

8. Flight control apparatus for an aircraft having a control surface and power means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for `said operating means, said first control means including a first controller responsive to the changes in air speed and a second follow-up controller driven by said first operating means and a third controller responsive to the change in position of said aircraft about 4an axis; a second operating means for governing the power of said power means; a second control means for said second operating means, said second control means including a fourth controller responsive to the change in altitude of said aircraft and a fifth follow-up controller driven by said second operating means.

9. Flight control apparatus for an aircraft having a control surface and power means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for said operating means, said rst control means constituting a rebalanceable network including a plurality of series connected variable impedances; a first controller responsive to changes in air speed connected to one impedance; a second controller driven by said operating means; and a third controller responsive to the movement of the aircraft about an axis; a second operating means for controlling the power of said power means; a second control means for said second operating means, said second control means constituting a rebalanceable network including a plurality of series connected variable impedances; a fourth controller responsive -to changes in altitude connected to one impedance in said second control means; and a fifth controller driven by said second operating means connected to another impedance in said second control means.

1G. Flight control apparatus for an aircraft having a control surface and power means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for said operating means, said first control means including a first controller responsive to changes in altitude of said aircraft, a second controller responsive to the changes in air speed of said aircraft, and a third or followup controller driven by said first operating means, a second operating means for controlling the power of said propelling means; a second control means for said second operating means, said second control means including a fourth controller responsive to changes in the altitude of said aircraft, a fifth controller responsive to changes in air speed of the aircraft, and a sixth controller driven by said second operating means.

l1. Flight control apparatus for an aircraft having a control surface and power means for propelling said aircraft comprising: a first operating means for said control surface; a first control means for said operating means said first control means including a first controller responsive to changes in altitude of said aircraft, a second controller responsive to the changes in air speed of said aircraft, and a third or followup controller driven by said operating means; a second operating means for controlling the power of said power means; a second control means for said second operating means, said second control means including a fourth controller responsive to changes in air speed of said aircraft and a fifth controller or follow-up controller driven by said second operating means.

l2. Flight control apparatus for an aircraft having a power means for propelling said aircraft comprising: a first operating means for controlling the power of said power means; a first control means for said operating means said control means constituting a rebalanceable circuit including a plurality of series connected variable impedances; means responsive to changes in altitude of the aircraft connected to one impedance; means responsive to changes in air speed of said aircraft connected to a second impedance; means for rendering one responsive means effective and the other ineffective; and follow-up means driven by said operating means and connected to a third variable impedance.

13. Flight control apparatus for an aircraft having a control surface for controlling the movements of the aircraft about an axis and power means for propelling said aircraft comprising: a rst operating means for said control surface; a first control means for said operating means; a second operating means for controlling the power of said power means; a second control means for said second operating means; each of said control means including a balanceable network comprising a plurality of series connected variable voltage sources; means responsive to changes in altitude of said aircraft to vary a first voltage source in each network; means responsive to changes in air speed of the aircraft to vary a second voltage source in each network; and follow-up means driven by each operating means to vary a third voltage source in each network.

14. in control apparatus for an aircraft having a control surface means for controlling the position of the aircraft about an axis and power means operating on variable fuel air ratios for propelling said aircraft including means for varying said ratio to said power means: a first operating means for said control surface means, a first control means for said operating means; a second control means for said ratio varying means, each said control means including a rebalanceable system; means responsive to changes in altitude and air speed of said aircraft to unbalance each system; means driven by said first operating means for rebalancing one system; and means moved in accordance with changes in fuel ratios for rebalancing said other system.

l5. Control apparatus for an aircraft having a control surface means for controlling the movement of the aircraft about an axis and power means for propelling said aircraft comprising; a rst operating means for said control surface means; a first control means for said operating means; a second operating means for controlling the power of said power means; a second control means for said second operating means; each of said control means comprising a rebalanceabie system and including means responsive to changes in altitude and air speed of said aircraft; one control means also including means responsive to movement of the aircraft about the roll axis and a rebalancing controller driven by said rst operating means, the other control means also including a rebalancing controller driven by the second operating means.

-16. In combination with an aircraft having an elevator control surface and power means for propelling said aircraft: an altimeter; control surface actuating motor means; means for controlling the power of said propelling means; differential means operated by said altimeter upon decrease in altitude for controlling said motor means to position said lsurface in a direction tending to increase the elevation of said aircraft; further means concurrently operated by said altimeter and connected to the power control means for increasing the power of said propelling means; air speed responsive means; further means in said differential means operated Iby said air speed means upon decrease in speed of the craft for controlling said motor means to move the elevator in a direction tending to decrease the altitude of the aircraft; and additional means concurrently operated by said air speed means and connected to said power control means to increase the power of said propelling means.

17. Flight control apparatus for an aircraft having an elevator control surface and a power means for propelling said aircraft, said apparatus comprising: operating means for said control surface; a rst balanceable control means for effecting operation of said operating means, said control means including an initiating means responsive in proportion to the altitude changes of said aircraft and unbalancing said control means proportionately to the altitude change and a follow up means driven by said operating means in accordance with the magnitude of unbalance whereby said control means may be rebalanced and said surface positioned; a second balanceable control means for effecting changes in output -of said power means in accordance with its unbalance including an initiating means responsive in proportion to the altitude changes of the aircraft and unbalancing said second control means proportionately to the altitude change and a follow-up means responsive to changes applied to said power means whereby said second control means is balanced, whereby said control surface is displaced to compensate for any tendency of the craft to change air speed due to changes in output of said power means to remove the altitude change; and means in said rst control means to modify the extent of unbalance thereof caused by said altitude responsive means to accurately provide such control surface compensation.

18. Flight control apparatus for an aircraft having an elevator control surface and power means for propelling said aircraft, said apparatus comprising: operating means for said control surface; a rst balanceable control means for said operating means; a second balanceable control means for controlling the power output of said power means, said first and second control means including means proportionately responsive to the magnitude of change in air speed of the aircraft from a predetermined value to effect proportionate unbalance in both said control means; means driven by said operating means to rebalance said first control means and to position said elevator in proportion to change in air speed; means for rebalancing said second control means and adjusting the output of said power means in proportion to the change in air speed, whereby said power means tends to compensate for any tendency to change altitude due to the displacement of said elevator to maintain the predetermined air speed; and manually adjustable means in said second control means to modify the extent of unbalance thereof caused -by said speed responsive means to provide such accurate power output compensation.

FRED P. STROTHER.

REFERENCES CITED The following references are of record-in the le of this patent:

UNITED STATES PATENTS 

